In a clinical high dose rate (HDR) brachytherapy catheters are inserted into a target object within a person, wherein through the inserted catheters radiation sources are introduced into the target object for treating the same. For determining the three-dimensional pose and shape of the catheters within the person, a user introduces sequentially a guidewire into the catheters, wherein the position of the tip of the guidewire within the respective catheter is electromagnetically tracked. The tracked position of the tip of the guidewire is acquired under the control of the user. However, since the catheters are inserted into the person to different depths, it is very difficult for the user to acquire the tracked position at appropriate locations within the respective catheter. Thus, generally unnecessary data are acquired, for instance, data corresponding to locations outside the person, which have to be identified correctly and filtered out, thereby rendering the complete procedure relatively cumbersome.